One of the oldest methods of extracting energy from fluids in motion is by means of bladed rotating machines such as, for example, windmills exposed to the wind and hydro impellors powered by the fluid reaction of moving water. While these devices have been successfully employed for many hundreds of years, there still remain major unresolved technical problems in the design of conventional wind turbines which make them problematic for small and large scale energy production.
These technical problems fall into basically three categories: fluid dynamics, dynamic stress, and electrical conversion.
The fluid dynamic difficulties can best be appreciated by the Betz theory which expresses limitation on potential energy conversion and which is described in Technical Note #75 "Meteorological Aspects of Utilization of Wind as an Energy Source," World Meteorological Organization 1981. The column of air (wind) impelling upon the windmills blades is slowed and its boundary is an expanding envelope where streamlines meet turbulence behind the rotor. Attempts to shroud the envelope to utilize the lower pressure behind the blades as a fluid dynamic advantage have been suggested in U.S. Pat. No. 4,075,500 as well as others. To date no practical cost effective method exists for fabricating ducted shrouds, much less balance them on support structures.
Mechanical stresses induced on the blading and support structure present further limitations especially for large windmills. On the supporting structure the axial stress representing the force which tends to overturn the windmill, or the thrust on the bearing, must be kept within limits at all wind speeds. To accomplish these results and to generate sufficient and efficient power, large diameter blades with built-in governors for adjusting the pitch angles of the blades have been utilized. These mechanisms proposed to date make the blades fragile and costly.
Furthermore, large diameter blades, such as over 100 feet in length, present significant dynamic stress problems. The increased blade length of larger rotors requires greatly increased blade stiffness and reduced weight in order to insure that critical vibrational frequencies of the blade remain sufficiently outside the excitation frequencies associated with routine operation so that the blades do not become unstable. A combination of gravitational force and torque force on each blade element functions to cyclically stress the blade element as it rotates in a rising direction and then a falling direction. Long blades supported at their roots and under the influence of the aforementioned oscillating forces are subjected to an increasingly severe and complex system of dynamic instabilities. It is difficult and expensive to safeguard against such instabilities. Blade stiffness to weight ratio improvements and advanced design methods can help but there is always a practical maximum to the size of blades being employed by wind turbines.
Finally, conventional wind turbines generate power through a gearbox and conventional generator. Although these generators are commonly available, they represent a one hundred year old technology that has not incorporated recent technological breakthroughs in material science, power control and aerodynamics. These generators, while compact, are heavy because of the massive laminated iron core and copper windings.